HUBBLE DISCOVERS MISSING PIECES OF COMET LINEAR

To the surprise and delight of astronomers, NASA's Hubble
Space Telescope has discovered a small armada of "mini-comets"
left behind by what some astronomers had assumed was a total
disintegration of the explosive comet LINEAR.

Hubble's powerful vision has settled the fate of the
mysteriously-vanished solid nucleus of the comet, which seemed to
disappear after it moved around the Sun.

On July 27, ground-based observers lost sight of the bright
core of the comet and suggested that the nucleus disintegrated
into a pile of dust. Astronomers at the Space Telescope Science
Institute (STScI) in Baltimore, MD, quickly reprogrammed Hubble to
search for the missing nucleus. Johns Hopkins University
astronomer Hal Weaver said he was stunned when the Hubble image
popped up on his computer screen. "My first thought was Hubble
Space Telescope does it again! We caught the fish! This is
amazing, very exciting, very neat."

Though comets have been known to break apart before, this is
the first time astronomers have a close-up view of the dismantling
of a comet's nucleus due to the Sun's heat. Since the 1950s,
researchers assumed comet nuclei were loose clusters of ice and
dust, called cometesimals, held together by gravity. Solar heat
causes the ices to sublimate and violently release gas as
explosions and garden hose-style jets. The pressure of the solar
radiation blows away particles like debris caught in a gale.

Some astronomers think that the fragments now being seen in
LINEAR may be the primordial building blocks of the original
nucleus, the so-called cometesimals, which theory predicts should
be several tens of feet across. The breakup of a comet tells
scientists how it was put together in the first place. The
cometesimals were built up from micron-sized grains of dust as it
collected in the early solar system, roughly 4.6 billion years
ago.

On Weaver's screen were at least a half dozen "mini-comets"
with tails, resembling the shower of glowing fireballs from
fireworks. They were clustered in the lance-head tip of an
elongated stream of dust and an isolated brighter piece in front
of the cluster may be the parent nucleus for the smaller
fragments. Hubble's exceptional resolution and sensitivity allowed
it to reveal the nuclei as separated bodies at a level of detail
never before seen in a disintegrating comet.

Some astronomers find it hard to imagine how an object the
size of a mountain could totally disintegrate in only two weeks.
"Actually, I would have been more amazed if Hubble saw no pieces,"
adds co-investigator Carey Lisse, of STScI. "The comet's breakup
was too violent and fast for it to completely vaporize. How do you
pulverize something the size of a mountain?"

Weaver says it will be important for the largest ground-based
telescopes to try and see the mini-comets as they spread apart.
This may yield further clues on the structure of the original
nucleus and the sizes of the remaining fragments.

Some astronomers believe this was Comet LINEAR's first visit
to the inner solar system, after traveling for nearly the distance
of one light-year (six trillion miles) from the vast comet
storehouse called the Oort cloud. Other astronomers suggest that
LINEAR may have been a fragile piece that broke off of a larger
comet that visited our solar system more than 10 million years
ago.

It's estimated that 20-30 percent of comets are so fragile
they completely disintegrate when they pass the Sun.

The Space Telescope Science Institute is operated by the
Association of Universities for Research in Astronomy, Inc., for
NASA, under contract with NASA's Goddard Space Flight Center,
Greenbelt, MD. The Hubble Space Telescope is a project of
international cooperation between NASA and the European Space
Agency (ESA).

Comet LINEAR blew apart so thoroughly last week that astronomers can't
find any sizable pieces from its fractured icy core. New groundbased
images of the comet posted today revealed no fragments brighter than 22nd
magnitude. The new data are fueling speculation that Comet LINEAR itself
might have been the fragment of a larger body that passed through the
inner solar system centuries ago.

COMET LINEAR: GOING, GOING... BUT NOT QUITE GONE !

Continuous monitoring of Comet LINEAR by the Jacobus Kapteyn Telescope
showed that it was disintegrating as it approached the Sun. The latest and
best images from the larger aperture Isaac Newton Telescope now give us the
clearest idea so far of how and why the comet disintegrated.

Having previously appeared completely normal, on the night of July 25th the
comet was seen to undergo a rapid change. The initially compact comet
nucleus evolved into a fuzzy, extended and much fainter object. This caused
much speculation as to what the reason for the disruption of the comet might
be. Further observations with the telescopes of the Isaac Newton Group at
the Roque de los Muchachos Observatory, La Palma, Spain as well as
telescopes elsewhere have confirmed the initial discovery and provided new
insight into what the reason for the comet disruption could be: the
evaporation of all the ice in the nucleus.

Cometary nuclei are a mixture of solid lumps of material of various sizes,
held together by a cement of ices. When comets pass close to the Sun during
their journey across the solar system the icy elements (mainly water ice and
carbon monoxide ice) sublime, leaving loose material behind that forms the
dust tail of the comet, while the sublimed ice forms its gas tails. As a
result of this process, or due to the strong gravitational pull from a
planet such as Jupiter, or from the Sun, a comet nucleus may sometimes split
into two or more fragments. What was seen in the case of Comet LINEAR,
however, was different.

From analysis of the images that lay out the recent sequence of events for
Comet LINEAR, Dr. Mark Kidger from the Spanish Instituto de Astrofisica de
Canarias concludes that this small comet probably ran out of ice altogether,
leaving behind a loose conglomerate of particles that are now gradually
dispersing into space. This model fits the observations well, as
measurements have shown that the activity of the comet had been declining
for several weeks as ice gradually sublimed away. During the comet's closest
approach to the Sun, a burst of activity was recorded. Then, when all the
ice was exhausted and nothing was holding together the solids, the nucleus
began to fall apart.

The latest images taken with the 2.5 metre Isaac Newton Telescope after
break-up show no sign of the comet's original nucleus, nor of any active
sub-nuclei larger than a few metres across. Any large remnants of the
nucleus that remain cannot be subliming significantly or they would have
been detected in these images. This corroborates the sequence of events
proposed by Dr. Kidger. Other comets are known to have disappeared, but
Comet LINEAR is the first one to have been caught in the act.

Comet LINEAR has maybe not been a spectacular night-time sight for most
people, but for astronomers it presents an important and unique event of
what can be described as the death of a comet.

The Isaac Newton Group of telescopes (ING) is an establishment of the
Particle Physics and Astronomy Research Council (PPARC) of the United
Kingdom and the Nederlandse Organisatie voor Wetenschappelijk Onderzoek
(NWO) of the Netherlands. The ING operates the 4.2 metre William Herschel
Telescope, the 2.5 metre Isaac Newton Telescope, and the 1.0 metre Jacobus
Kapteyn Telescope. The telescopes are located in the Spanish Observatorio
del Roque de Los Muchachos on La Palma which is operated by the Instituto de
Astrofisica de Canarias (IAC).

PICTURES

Caption: This image, obtained on 1 August, is a 100-second exposure with the
Wide Field Camera of the 2.5 metre Isaac Newton Telescope. This section of
the full image measures 4.5 arcminutes, equivalent to 110,000 km at the
comet. This image is processed to show faint details in the coma of the
comet. This is by far the best and deepest image of the broken-up nucleus
that exists and gives us a clear idea of how and why the comet
disintegrated. No features are seen in the image, which implies that no
significant individual fragments more than a few metres across still emit
gas. This demonstrates the catastrophic disruption of the nucleus. Unlike
comet Shoemaker-Levy 9, the only remnant that remains is an expanding dust
cloud.

This image, obtained on 1 August with the Wide Field Camera of the
2.5 metre Isaac Newton Telescope, covers a field of view of 22 arcminutes
and is processed to show the faint tail of the comet, which extends well
beyond the edge of the field of view.

Comet LINEAR continues to disintegrate and could disappear completely within
a few days.

NASA Science News

July 31, 2000 -- Astronomers around the world continue to monitor the
unexpected disintegration of comet C/1999 S4 (LINEAR). Intense solar heating
apparently triggered a massive disruption of the comet's fragile icy core
when it passed close to the Sun last week. It is still bright enough to see
through small telescopes so even amateur astronomers can watch the comet as
it dissolves. If you do plan to look, don't wait. Experts think that comet
LINEAR might disappear completely in a few days.

The break up of a bright comet is unusual but not unprecedented. For
example, comet Shoemaker-Levy 9 (SL-9) broke up before it struck Jupiter in
1994. SL-9 was discovered after it fragmented, so there is no record of what
happened as it came to pieces. With comet LINEAR, astronomers have a
ringside seat for the entire show.

"We have observed a few comets in the process of breaking up -- comet West
in 1976, comet Ikeya-Seki in 1965 and others -- but never with so much
detail as we're seeing in comet LINEAR," says Mark Kidger, an astronomer at
the Instituto de Astrofisica de Canarias. Comet LINEAR's demise seems to be
a bit unusual. "Cometary splittings rarely ever lead to the rapid
disappearance of a comet like this - in fact, I don't know of another case"

Kidger was the first to notice comet LINEAR disintegrating as he monitored a
cloud of gas (called the "coma") surrounding the comet's core using the
1-meter Jacobus Kapteyn Telescope. Comet LINEAR, which has been falling
toward the Sun since it was discovered in September 1999, made its closest
approach to our star (perihelion) on July 26, 2000. Perihelion is a critical
time for any comet. It's when solar heating of the icy core is most intense
and when the comet swings around for its long return trip to the outer solar
system.

"At perihelion there are very rapid aspect changes as regions of the nucleus
previously in shadow are suddenly subjected to intense heating," continued
Kidger. "This causes strong thermal stresses" that may have been a primary
cause of LINEAR's breakup.

Something was already amiss the day before Comet LINEAR reached perihelion
at a distance of 114 million km (0.74 AU) from the Sun.

"The very first images on July 25th were enough to show me that something
odd was going on," recounts Kidger. "The comet's inner coma was no longer
teardrop-shaped (the solar wind flowing around the comet's head causes this
shape). It had a shape like a short, fat cigar. My first thought was
'Shoemaker-Levy.' It looked just like those first images of Comet
Shoemaker-Levy 9 after it was discovered."

Kidger's images on subsequent nights confirmed that something dramatic was
happening and he announced his findings in an International Astronomical
Union (IAU) Circular (IAUC #7467) on July 27, 2000. As news of the breakup
spread, astronomers around the world trained their telescopes on the comet.
In another IAU Circular (IAUC # 7468) published July 28th, three teams of
observers reported that they too saw evidence of a major event in the
comet's nucleus.

Unlike comet Shoemaker-Levy 9, which broke into many well-defined bright
fragments, comet LINEAR seems to be dissolving into an amorphous haze of gas
and dust.

"There is some similarity of appearance to the two comets," says Brian
Marsden of Harvard's Minor Planet Center. "An observation by Ian Griffin in
New Zealand on July 29th shows the nucleus of C/1999 S4 (LINEAR) extended
into a long, bright string. However, it does not seem to show discrete
nuclei in that string, as D/1993 F2 (SL-9) did."

The differences between comets SL-9 and LINEAR result from their different
sizes and distances from the Sun.

Comet Shoemaker-Levy 9 was larger than comet LINEAR, and it broke apart as
the result of tidal stresses it experienced when it passed less than 100
thousand kilometers from Jupiter (within 1.4 Jupiter radii from the planet's
center). SL-9 was far from the Sun (812 million km) when it fragmented and
solar heating was not the primary cause of the break up. In fact, SL-9
wasn't even orbiting the Sun. The comet had been captured by the
gravitational pull of Jupiter and was orbiting the giant planet instead.

Comet LINEAR is a much smaller object that has been losing mass rapidly
during its approach to the Sun. The Hubble Space Telescope recorded a
house-sized fragment blowing away from the core on July 5th and powerful
jets of gas vaporized by solar radiation have been pushing the comet to and
fro. Solar heating is a more important factor in its breakup than
gravitational effects.

"The small size of comet LINEAR and its exposure to solar radiation is
causing a more complete and rapid dissolution than we saw in Shoemaker-Levy
9," continued Marsden. "The initial break-up of SL-9 was surely caused by
tidal forces from Jupiter. If they had not later collided with Jupiter,
several of those fragments would presumably still exist. C/1999 S4 (LINEAR),
on the other hand, will probably have completely dispersed in a week or so."

Comet LINEAR may still be bright enough for amateur astronomers to view in
small telescopes, but it's fading fast. On July 27th, binocular observers in
South America and Europe estimated the comet's visual magnitude to be +6.6.
That's almost bright enough to see with the unaided eye from dark-sky
observing sites. Two days later, an experienced amateur in the Canary
Islands reported a visual magnitude of +8.3, a factor of 6 decline in
brightness.

"The surface brightness of the innermost coma is fading fast," says Kidger.
"This should translate to a somewhat slower fade of the outer coma [that
binocular and small telescope observers see] as the gas and dust in it
disperses and is not replenished. Typically a comet may take several weeks
for the coma to expand and fade down to the brightness of the sky
background."

Many well-known annual meteor showers, including the Perseids, Leonids and
Geminids, are caused by dusty debris from comets burning up in the
atmosphere of Earth. Such displays are harmless and beautiful. Unfortunately
for meteor lovers, the orbit of comet LINEAR comes no closer to our planet
than 28 million kilometers (0.18 AU). There will be no "Linearid" meteor
shower. When comet LINEAR finally disappears from view in a few days or
weeks, this memorable visitor from beyond the orbit of Neptune will be gone
forever.

Space Weather News for July 28, 2000

There is growing evidence that comet LINEAR, which made its closest
approach to the Sun earlier this week, is disintegrating. Today's
spaceweather.com features images and animations of the apparent breakup.

Although comet LINEAR was not bright enough to
see with the unaided eye when it passed by Earth
this week, the comet is grabbing the attention of
astronomers with peculiar behavior, including
orbit-altering jets and fragments breaking away
from its nucleus.

DEATH OF A COMET: LINEAR BLOWN APART

Astronomers report that Comet LINEAR, the brightest comet of the
year, appears to have blown apart. The comet, which at one time was
predicted to become visible to the naked eye, dramatically faded this
week.

NASA OBSERVATORIES KEEP THEIR 'EYES' ON LINEAR

When NASA's two great observatories, Hubble and Chandra, recently
observed comet LINEAR astronomers received some abrupt surprises.
Researchers were able to catch the icy comet in a brief, violent
outburst when it blew off a piece of its crust, like a cork popping
off a champagne bottle.

THE JACOBUS KAPTEYN TELESCOPE OBSERVES THE DEATH OF COMET LINEAR

Nightly observations made since July 23 in different broadband filters with
the 1-m Jacobus Kapteyn Telescope on La Palma, Canary Islands, Spain show
what appears to be the complete disruption of the nucleus of comet LINEAR,
the brightest comet of the year.

Dr. Mark Kidger, Instituto de Astrofisica de Canarias reports from the
Jacobus Kapteyn Telescope: "The central condensation was highly condensed
and showed the typical 'teardrop' form in the evening of July 23rd and July
24th, although its brightness decreased by a factor of about 3 between the
two nights. In the evening of July 25th something very odd was happening to
the comet: the central condensation was seen to be strongly elongated, with
a very flat brightness distribution. The condensation's brightness faded
further and its length increased on the following nights. On July 27 there
was no evidence of any local brightness peaks that would indicate the
presence of sub-nuclei."

In other words, it does not appear to have broken into individual fragments
in the way that Comet Shoemaker-Levy 9 did in 1993. Instead, it has
completely blown apart.

The expansion velocity of the condensation is about 40 m/s, indicating that
it is solid particles and not gas. The gas tail, which virtually disappeared
between July 23rd and 24th, has reformed as an extension of the major axis
of the central condensation.

Comet LINEAR, or C/1999 S4 (LINEAR) as it is called in correct astronomical
nomenclature, is a by-product of the automated LINEAR minor-planet survey.
Discovered nearly as far out as Jupiter last September, this comet passed
114 million kilometres from the Sun on July 26 and only 56 million
kilometres from Earth on July 22. Comet LINEAR is a "new" comet which means
that it is making its very first passage through the inner solar system. The
surfaces of new comets are believed to be covered almost completely by a
very thin, fragile layer of highly volatile ices such as carbon dioxide
intermixed with dust.

When discovered, Comet LINEAR was immediately regarded as a candidate likely
to reach naked eye visibility based on its relative brightness and large
heliocentric distance. New comets though are notoriously difficult to
predict as far as their light curve behaviour is concerned, particularly
many months in advance.

At present Comet LINEAR is diving southward from the constellation Ursa
Major into Leo. It will be about 20 degrees above the west-northwest horizon
as evening twilight deepens. Although not visible to the naked eye, the
comet will continue to be in northern skies until the second week of August,
when it will dip below the horizon. The observations with the Jacobus
Kapteyn Telescope suggest though that the comet is dying very quickly and
may disappear completely within a few days.

The Jacobus Kapteyn Telescope will continue to observe its disintegration
over the next few nights, hopefully giving new insights into the nature of
comet nuclei and their structure.

The Jacobus Kapteyn Telescope is part of the Isaac Newton Group of
Telescopes (ING). The ING is an establishment of the Particle Physics and
Astronomy Research Council (PPARC) of the United Kingdom and the Nederlandse
Organisatie voor Wetenschappelijk Onderzoek (NWO) of the Netherlands. Apart
from the Jacobus Kapteyn Telescope, the ING also operates the 4.2 metre
William Herschel Telescope and the 2.5 metre Isaac Newton Telescope. The
telescopes are located in the Spanish Observatorio del Roque de Los
Muchachos on La Palma which is operated by the Instituto de Astrofisica de
Canarias (IAC).

PICTURES

Caption: An image of the nucleus of Comet LINEAR as seen on the Jacobus
Kapteyn Telescope on July 26th. This is the raw, unprocessed image that the
astronomer saw straight off the telescope. The unusual elongated shape was
the first evidence of the comet's complete break-up.

Using NASA's Hubble Space Telescope, researchers were surprised to
catch the comet LINEAR (C/1999 S4) in a brief, violent outburst when
it blew off a piece of its crust, like a cork popping off a champagne
bottle. The eruption, the comet's equivalent of a volcanic explosion
(though temperatures are far below freezing, at about minus 100 degrees
Fahrenheit in the icy regions of the nucleus or core), spewed a great
deal of dust into space. This mist of dust reflected sunlight,
dramatically increasing the comet's brightness over several hours.
Hubble's sharp vision recorded the entire event and even snapped a
picture of the chunk of material jettisoned from the nucleus and
floating away along the comet's tail.

NASA'S TWO GREAT OBSERVATORIES KEEP THEIR "EYES" ON COMET LINEAR

When NASA's two great observatories, the Hubble Space
Telescope and the Chandra
X-ray observatory, recently observed comet LINEAR (C/1999 S4)
astronomers received some abrupt surprises.

Using the Hubble Space Telescope, researchers were surprised
to catch the icy comet in a brief, violent outburst when it blew
off a piece of its crust, like a cork popping off a champagne
bottle.

The eruption, the comet's equivalent of a volcanic explosion
-- though temperatures are far below freezing (about minus 100
degrees Fahrenheit or minus 40 degrees Celsius) in the icy regions
of the nucleus or core -- spewed a great deal of dust into space.
This mist of dust reflected sunlight, dramatically increasing the
comet's brightness over several hours. Hubble's sharp vision
recorded the entire event and even snapped a picture of the chunk
of material jettisoned from the nucleus and floating away along
the comet's tail.

"We lucked out completely," said Hubble comet-watcher Harold
Weaver of the Johns Hopkins University, Baltimore, MD. "In one
surge of brilliance this under-performing comet showed us what it
could have been. Comet LINEAR generally has not been as bright as
we had hoped, but occasionally does something exciting."

Though comet nuclei have been known to fragment, Hubble's
sharp vision is revealing finer details of how they break apart.
This unexpected glimpse at a transitory event may indicate that
these types of "Mt. Saint Helens" outbursts occur frequently on
the comet, because it is unlikely that Hubble just happened to
catch one isolated event, Weaver said.

The orbiting observatory's Space Telescope Imaging
Spectrograph tracked the streaking comet for two days, July 5 to
7, capturing the leap in brightness and discovering the castaway
chunk of material sailing along its tail. When the Hubble
telescope first spied the comet 74 million miles (120 million km)
from Earth, it watched the icy object's brightness rise by about
50 percent in less than four hours. By the next day, the comet was
a third less luminous than it had been the previous day. On the
final day, the comet was back to normal.

During the outburst's peak, the astronomers believe that the
comet jettisoned the piece of its crust seen days later in the
tail. The renegade fragment moved away from the core's weak
gravitational grasp at an average speed of about six miles per
hour, which is a little more than a brisk walking pace.

A week later, on July 14, NASA's Chandra X-ray Observatory
imaged the comet and detected X-rays from oxygen and nitrogen
ions. The details of the X-ray emission, as recorded on Chandra's
Advanced CCD Imaging Spectrometer (ACIS), show that the X-rays are
produced by collisions of ions racing away from the Sun with gas
in the comet.

"This observation solves one mystery. It proves how comets
produce X-rays," said Carey Lisse of the Space Telescope Science
Institute, Baltimore, MD, leader of a team of scientists from the
institute; NASA's Goddard Space Flight Center, Greenbelt, MD;
Johns Hopkins; the University of California, Berkeley; and the
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA. "With
an instrument like Chandra, we can now study the chemistry of the
solar wind and observe the X-ray glow of the atmosphere of comets,
as well as other planets such as Venus."

Comet LINEAR was named for the observatory that originally
discovered it in September 1999. LINEAR is the acronym for Lincoln
Near Earth Asteroid Research, a project operated by the
Massachusetts Institute of Technology's Lincoln Laboratory,
Lexington, MA, to search for Earth-approaching objects.

The Space Telescope Science Institute is operated by the
Association of Universities for Research in Astronomy, Inc., for
NASA, under contract with Goddard Space Flight Center. The Hubble
Space Telescope is a project of international cooperation between
NASA and the European Space Agency.

Comet LINEAR will reach maximum brightness around July 23, 2000, as it
glides past the bowl of the Big Dipper. Sky watchers have been hoping
that LINEAR would become visible to the unaided eye. However, monitoring
data from a global network of astronomers suggest that the comet's
brightness will peak at a visual magnitude of +6.5, just below the
threshold for naked-eye visibility. LINEAR should still be a visual treat
when viewed through binoculars or a small telescope.

For more information please visit
SpaceWeather.com
NOTE to readers: Since SpaceWeather.com was launched in its current form
on January 1, 2000, the site has focused on solar and geomagnetic
activity. Comets and meteors are an important aspect of space weather,
too. With today's update about comet LINEAR we will begin an accelerating
program of coverage for comets, meteor showers, and related astronomical
events.

COMET LINEAR (C/1999 S4)
This summer's well-known cometary visitor is having its last gasp and has
all but disappeared from view. On July 24th the cometís strongly condensed
nucleus began to elongate, and its brightness diminished dramatically. Most
likely, the comet broke apart as it approached its July 26th perihelion,
when it was closest (114 million kilometers) to the Sun. At that time the
nucleus would have undergone incredible thermal stress, and it appears
Comet LINEAR buckled and disintegrated under the intense heat. This week
astronomers plan to use the Hubble Space Telescope to look for fragments of
the disrupted nucleus. Meanwhile, for telescopic observers here on Earth,
the comet has begun to move south and had faded to below 9th magnitude.

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Comet LINEAR Bright Enough For Binoculars

By Jeff Kanipe
space.com

23 June 2000

A comet heading for the inner solar system may not fulfill expectations of
being bright enough to be seen with the naked eye, but astronomers are at
least sure of one thing: it will be the brightest comet since Hale-Bopp
graced the heavens three years ago. You can add another thing. It should be
bright enough to seen with binoculars.

The new comet goes by the mumbo-jumbo name of 1999 S4 LINEAR, an acronym
which stands for Lincoln Laboratory Near-Earth Asteroid Research -- an
automated-search program in New Mexico that images regions of the night sky
looking for asteroids and comets. This one was found in digital images made
on September 27 and was reported, at the time, as "an unusual moving
object."

Icy visitor may become visible to naked eye

By Jeff Kanipe

June 6, 2000

The buzz among amateur astronomers for the last several months concerns a
comet that may reach naked-eye brightness in mid July. Comet 1999 S4 LINEAR
(named after Lincoln Laboratory Near Earth Asteroid Research, an automated
program that images regions of the night sky looking for asteroids and
comets) may reach anywhere between magnitudes 3 and 5 or 6 around July
19-25.

Although not a blockbuster by any means, it's still potentially the
brightest comet we've had since Hale-Bopp's momentous apparition three years
ago.

Comet LINEAR may hover between magnitude 5 and 6 until early August. Right
now, it's a small, magnitude-10 fluff of light within the constellation
Triangulum in the predawn sky. A 4-inch (10-centimeter) telescope and low
magnification should be able to detect it. You might even try binoculars.